According to EP 0 198 684 B1, a differential relay operates to protect an electric power transmission line which is monitored at various points with regard to the current flowing through. The currents at the various points on the power transmission line are converted into digital input signals by using a dedicated operating clock in each case to sample the power supply line to be monitored at the various points. The sampling is undertaken in this case at the various points not with the aid of a synchronous clock, but with the aid of weakly differing clock frequencies. Running between the various points of the power supply line is a digital transmission channel via which a calling message is transmitted by a detecting device (master) at one point of the power supply line to another point, the calling message also including data which give information on the sampling instant at the one point. In response to the calling message, a detecting device (slave) at the other point of the power supply line emits a return signal which includes, inter alia, the information on the sampling instant in the master and on a time difference between the last sampling instant in the slave and the subsequent receiving instant of the calling message in the slave. The return signal received by the master is used in the master to draw a conclusion on the temporal skew of the sampling instants at the two different points on the power supply line, and the time skew is compensated with regard to the various sampling instants after a vector transformation of the received data by means of an appropriate pointer rotation.